Abstract Classes in Java

Data abstraction is the process of hiding certain details and showing only essential information to the user.

Abstraction can be achieved with either abstract classes or interfaces

The abstract keyword is a non-access modifier, used for classes and methods:

• Abstract class: is a restricted class that cannot be used to create objects (to access it, it must be inherited from another class).
• Abstract method: can only be used in an abstract class, and it does not have a body. The body is provided by the subclass (inherited from).

Following are some important observations about abstract classes in Java.

• An abstract class must be declared with an abstract keyword.
• An abstract class can have abstract and non-abstract methods.
• An abstract class cannot be instantiated.
• An abstract class can have constructors and static methods also.
• An abstract class can have final methods which will force the subclass not to change the body of the method.

Example:

abstract class Base {

            abstract void fun();

}

class Derived extends Base {

            void fun()

            {

                        System.out.println("Derived fun() called");

            }

}

class Main {

            // Main driver method

            public static void main(String args[])

            {

                        Base b = new Base(); // gives error if written, creating instance of abstract class.

                        Base b = new Derived();

                        b.fun();

            }

}